The electric power steering apparatus is a machine that operates the motor for generating the steering assist force according to a steering torque applied by a driver. In recent years, the demand for electric power steering apparatuses for large-size automobiles has increased rapidly. Such a large-size automobile requires an increased steering assist force so that a larger amount of electric power must be supplied to the motor. In some cases, however, a battery alone is unable to fully meet such a large power demand. In this connection, an arrangement has been proposed wherein an auxiliary power supply is provided in addition to the battery such that the normal power demand is met by the battery alone but when a larger amount of electric power is required, the electric power is supplied by both the battery and the auxiliary power supply connected in series (see, for example, Japanese Unexamined Patent Publication No. 2005-287222 (FIG. 1)).
The following configuration is commonly contemplated as the electric circuit of the above-described electric power steering apparatus that uses the auxiliary power supply as required. An electrical conduction path for supplying electric power from a source consisting of the series-connected battery and auxiliary power supply is provided as an independent system from an electrical conduction path for supplying the electric power from the battery alone. The two conduction paths are joined together at some midpoint and led to a motor driving circuit. Each of the conduction paths has a switching device such as a MOS-FET interposed therein. The two switching devices are so controlled as to be turned on alternately.
However, the following problem is encountered if a short-circuit fault (source-drain short circuit of the MOS-FET, for example) occurs in the switching device interposed in the conduction path for supplying the electric power from the battery alone. When a high-voltage power is supplied from the series-connected sources, a current induced by the voltage of the auxiliary power supply is reversed upstream from the junction of the conduction paths. The current flows in a closed circuit powered by the auxiliary power supply and including the two on-state switching devices (one of which is short-circuited). Therefore, the supply of high-voltage power is disabled while wasteful discharge is performed due to continuation of current-flowing. Subsequently when the power supply is returned to a normal mode relying on the battery alone, the fact of the failure is hard to be detected because the damaged switching device is substantially in the same state as the normal on-state.